1. Field of the Invention
The present invention relates to an interference film filter for a copying machine.
2. Description of the Prior Art
An interference film filter is known in the art, which includes a multi-layer film of a metal or non-metal formed by vapor deposition for preventing transmission of light in a selected wavelength range. This interference film filter has excellent cutting characteristics, and is therefore used as an orthochromatic filter for a copying machine. The filter is formed by alternately depositing materials having different refractive indices on a glass substrate surface or an image-forming lens surface.
In recent years the copying machine has become increasingly small and compact, and its optical system often employs an image-forming lens having a wide field angle to achieve compactness. Where an image-forming lens having a wide field angle is used, however, the incident angle of light impinging upon the interference film filter varies over a wide range. As shown in FIG. 11, for example, an image-forming lens L3 has a maximum field angle of 26.6 degrees where a document plane to be exposed has a maximum length of 300 mm (with a maximum document size being A3) and the image-forming lens L3 has a focal length of 150 mm. With an interference film filter mounted in an aperture or a barrel of this image-forming lens L3, on-axis light (i.e. a light beam that forms an image at the central point on a photoreceptor plane) and off-axis light (i.e. light beams that form images at marginal points on the photoreceptor plane) have spectral characteristics as shown in FIG. 12. As seen, the spectral characteristics of the off-axis light beams are such that light having wavelengths about 620 to 750 nm has reduced transmission factors, and therefore light having a wavelength with a transmission factor at 50%, for example, shifts about 30 nm to the shorter wavelength side, thereby forming a deviation from the spectral characteristics of the on-axis light. In this case, a copy image developed with black toner has a density difference up to 0.1 (as measured with a Macbeth densitometer) between its center corresponding to the on-axis light beam and the marginal ends corresponding to the off-axis light beams. A color copy image developed with a plurality of color toners has a color difference of 10 or more (as measured with a color difference meter Delta-E*ab) between the center and the marginal ends. Thus, non-uniformity occurs in the density and color.